Food starts to deteriorate or spoil from the time that it is harvested or slaughtered. The process commences with enzyme action which occurs as the cells in the food die. It continues with action by a range of spoilage bacteria and, in some foods, moulds and yeasts. The food will develop off flavours, odours and a breakdown in texture, providing recognisable signs that the food is no longer fit to eat.

In order to slow down the spoilage process and thus keep foods for a longer period, a wide range of preservation methods are applied to foods. Most methods involve the removal or control of factors which affect bacterial growth.

These include:

• temperature control - the use of low or high temperatures
• moisture control - dehydration
• use of chemicals
• physical methods.

In general, more than one method of preservation will be employed to achieve an acceptable shelf-life for products. A combination, for example, of some form of heat processing combined with the use of a chemical preservative, will often produce a more durable product than if a single preservation method were used.

The use of different preservation methods will be influenced not only by the shelf-life required but also by the demand for quality in terms of taste, smell, appearance, nutritional value, etc. of the food as some methods may significantly affect these aspects.

LOW TEMPERATURES
The use of low temperatures slows down the speed of enzyme reaction and also prevents the growth of most pathogens and food spoilage bacteria.

Preservation methods using low temperatures include:

• Refrigeration - storage between 0°C and 5°C.
• Freezing - storage at -18°C; there are a variety of commercial techniques to freeze foods which include fluidised-bed freezing, air-blast freezing, plate freezing, cryogenic freezing and the 'pellofreeze' system.

HIGH TEMPERATURES
Heat is used to destroy both pathogenic and spoilage bacteria; some heat processes will also destroy spores.

Preservation methods in this category include:

• Pasteurisation - time and temperature combination to destroy active pathogens and some spoilage bacteria in the product, e.g. for milk 72°C for 15 seconds.
• Sterilisation - destruction of all micro-organisms using temperatures above 100°C.
• Ultra-heat treated (UHT) - used for some products to destroy micro-organisms without causing significant change to flavour, relying upon high temperature for a very short time.

Preservation methods in this category include:

• Cooking - usually used to improve palatability rather than to improve storage quality.
• Ohmic heating - involves use of high-voltage electric current through foods to destroy micro-organisms.
• Canning - involves use of heat to appropriate temperature and for a prescribed time to destroy micro-organisms, including Cl. Botulinum spores. Process also includes removal of oxygen and hermetic sealing of containers to avoid post-process contamination.

Removal of moisture prevents micro-organisms from growing.

Preservation methods in this category include:

• Traditional methods such as sun-drying.
• Commercial methods using hot air including tunnel drying, roller drying and spray drying.
• Accelerated freeze-drying which uses warm air after food has been quickly frozen.

There are a wide range of chemical additives used for food preservation which affect the growth of micro-organisms in different ways.

Chemicals commonly used for food preservation include:

• Salt - makes water unavailable for use by micro-organisms.
• Sugar - acts in a similar manner to salt.
• Acetic and lactic acids - lower the pH below the normal growth range of most micro-organisms.
• Benzoic acid and sorbic acid - inhibit mould and yeast growth in acid foods.

The use of chemical preservative is the subject of specific regulations which control the types and quantity of chemicals which may be used in foods.

Different physical methods of preservation are used, including:

• Modified atmosphere packaging - air around the food product has a reduced oxygen content and increased level of nitrogen and carbon dioxide which will slow down growth of many micro-organisms.
• Vacuum packing - removes oxygen to inhibit growth of aerobic bacteria but does not prevent anaerobes from thriving. Often combined with refrigeration to give foods a reasonable shelf-life.
• Food irradiation - uses ionising radiation to destroy micro-organisms but not spores.